This invention relates to a conduit for the flow of a fluid and, more particularly, to a conduit including a low emissivity coating disposed to inhibit heat transfer through a wall of the conduit.
Power generating apparatus, for example an aircraft gas turbine engine, as well as various well-known flight vehicles, can be subject during operation to conditions producing undesirable atmospheric icing on components. For example, such icing can occur at and/or within such areas as an engine inlet or engine nacelle, as well as at various external surfaces of the vehicle. To reduce or eliminate such icing conditions, various anti-ice systems have been included in such apparatus.
In one form, a heated fluid, for example heated air from a turbine engine, is passed through a complex conduit, such as relatively long, spaced-apart double wall tubing including bends about other components, from a heat source to an area adjacent a potential icing site. For more efficient engine operation and use of such heated air, it is desirable to retain the heat in the air within the conduit during passage of the air from the heat source to the potential icing site. In addition, an outer surface of a heated anti-ice conduit potentially can be subject to contact with flammable fluids, one example of which is hydraulic fluid, leaking from another component and having an auto-ignition temperature lower than the temperature of the outer surface of the heated conduit. In that example, should a failure occur in a system adjacent the conduit, flammable fluid contacting an outer surface of an anti-ice conduit at a temperature at or above such auto-ignition temperature can result in a fire within the apparatus.
One current arrangement to reduce such flammable fluid ignition potential as well as to retain heat within the conduit is to dispose an insulating material about the conduit outer walls and associated supports. However, such insulation is relatively heavy, adding to the total engine weight. This reduces engine efficiency to the extent engine power is required to lift and carry such insulation. In addition, such insulation requires additional assembly time, is relatively expensive, reduces clearances, and is susceptible to field damage.
Various metallic and non-metallic coatings have been reported for use on external surfaces of components either for enhancing absorption of heat energy or for reflecting such energy from a surface. One type for thermal control surfaces of spacecraft is described in such U.S. Pat. No. 3,607,338—Webb and U.S. Pat. No. 5,296,285—Babel et al. Other types relate to hot operating components of turbine engines to reflect heat from an external surface of an article. One typical example is U.S. Pat. No. 5,851,679—Stowell et al. in which a multi-layer coating is provided on a substrate exposed to combustion gases.
Provision of a conduit wall with a coating that is very thin, light weight and effectively can inhibit heat transfer through the conduit wall can increase apparatus operating efficiency. In addition such a coating that can be provided on the conduit wall prior to assembly in an apparatus can reduce apparatus assembly time and thereby reduce the cost of the apparatus.